SL/PE1 ROM

Hello guys,
I am a bit confused about the specific cause for my incomplete ROM for Arch Body Holds. Basically I have pretty good back and shoulder flexibility (full bridge); in fact achieving shoulder flexion while performing the hold in not a problem for me. However my legs cannot get as high as demonstrated in the video. I was assuming that this was due to lack of hip flexor flexibility (I am about 7-8 inches off the floor in front splits) and I have been working on that. Anyway I asked this friend of mine with really loose hip flexors (oversplits) to try the Arch Body Hold and his legs were at the same level as mine.
This makes me kind of confused about what I need to do in order to master it. I keep striving to get better ROM every workout, but is there something specifically that has helped anyone achieve the proper ROM and master this? Thank you

I've had some success in increasing my leg range of motion by doing the following:

 

Lie on your stomach, tilt your pelvis posteriorly (so that your stomach lifts off the floor and your pubic bone presses into the floor)

checking with a mirror or video, make sure the line you create in your lower back stays exactly the same, and tense one side of your but so that the leg lifts off the floor. You can watch your lower back or touch it and make sure it is not tensing at all, so you are only using your gluts and hams.  I do ten lifts then hold for ten.  Then do the other leg.

 

I started doing this a couple of months ago and could barely lift my leg off the floor without my back engaging.  Now I can lift several inches and the strength in that range of motion seems to be carrying over to Arch Body Holds also.   

 

I think there have been several discussion about how passive flexibility does not carry over to active flexibility.   For example you might be able to lift your leg with your hands very high, when standing, but if you let go it drops back down.  Sounds like this is a similar issue and this exercise seemed to help me.